using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;

namespace Q.Framework.Extension
{
	/// <summary>
	/// A general utility method to compose lambda expressions without using invoke 
	/// (I’ll call it Compose), and leverage it to implement EF-friendly <c>And</c> and <c>Or</c> builder method
	/// </summary>
	public static partial class ExpressionExtension
	{
		public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
		{
			// build parameter map (from parameters of second to parameters of first)
			var map = first.Parameters.Select((f, i) => new
			{
				f,
				s = second.Parameters[i]
			}).ToDictionary(p => p.s, p => p.f);


			// replace parameters in the second lambda expression with parameters from the first
			var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);


			// apply composition of lambda expression bodies to parameters from the first expression 
			return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
		}

		public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.And);
		}

		public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.Or);
		}

	}
}